The present invention relates generally to telecommunication network systems, and more particularly, to a system and method implementing hard handoff for hierarchical cell sites within a Code Division Multiple Access (CDMA) telecommunication network.
One major feature of CDMA technology that differentiates it from other wireless communication technologies is its reuse of specific frequencies. Although it enjoys a technical edge over other communication technologies such as Time Division Multiple Access (TDMA) in many aspects, CDMA technology encounters an undesirable xe2x80x9chot spotxe2x80x9d problem. Since a comparatively large number of users may have to be serviced by one sector of a cell using a same frequency, such a heavily populated area makes up a xe2x80x9chot spotxe2x80x9d in the cell whereas the network performance deteriorates. Therefore, to ameliorate the problem of xe2x80x9chot spotsxe2x80x9d in modern telecommunication network system design, a cost effective hierarchical cell structure (HCS) is often desirable for increasing radio frequency (RF) channel capacity in a given location. A HCS normally refers to a system of overlay and underlay cells networked together to provide seamless communication for a mobile telecommunication unit (MU). Moreover, an overlay cell fully encompasses a smaller, underlay cell. In addition, an underlay cell can be enclosed by multiple levels of overlay cells. As shown in FIG. 1, a CDMA HCS usually comprises of an overlay cell 10 and at least one underlay cell 12.
However, because of the frequency reuse feature of CDMA technology, simply installing an underlay cell using the same carrier frequency as an overlay cell does not effectively eliminate a xe2x80x9chot spot.xe2x80x9d A soft handoff to the underlay cell may help to strengthen the signal quality in the hot spot, but does not improve overall capacity. Therefore, a hot spot may remain because no significant increase of users can be permitted, even with a HCS design. Moreover, problems relating to soft handoff and outside interference from the overlay cell could further degrade the network performance. Accordingly, an underlay cell using a different carrier frequency is desired.
U.S. patent application entitled xe2x80x9cPrecision Hard Handoff Method Using Pilot Beacon Distributors,xe2x80x9d which is identified as U.S. application Ser. No. 09/365,262 and assigned to the same entity, deals with a scenario that a specific underlay cell having a distinctive carrier frequency is installed to serve an isolated campus which has a large population of wireless MUs. Groups of distributed antennas are deployed and installed at each entrance of the isolated campus whereby MUs entering and exiting the campus are switched to an appropriate frequency of the covering cell. One drawback, among others, of this invention is that at least two antennas must be installed at every entrance or exit.
A method is needed for a better CDMA network design for embedding a CDMA cell or cells in an overlay cell for providing additional coverage and capacity for low mobility traffic without being obstructed by various interferences and problems caused by soft handoffs.
A system and method to implement a hierarchical CDMA network design is provided wherein at least one comparatively small underlay cell is embedded within a comparatively large overlay cell for the purpose of providing services to a large number of wireless users within certain areas of the overlay cell.
One embodiment of the present invention is directed to configuring an underlay cell within an overlay cell with both a traffic carrier and a pilot beacon or a xe2x80x9cLanternxe2x80x9d carrier. The Lantern carrier, which includes overhead channels such as Pilot, Paging and Sync, is programmed to set its overhead power level lower than the power level of the traffic carrier overhead channels so that the coverage or the footprint of the traffic carrier always encompasses the coverage of the Lantern carrier. Hence, the quality of a traffic signal is always assured when a mobile telecommunication unit (MU) is switched from the overlay cell to the underlay cell.
When an MU is active, i.e., it is engaging in a telephone call, and enters into the underlay cell, it will be switched to the underlay cell as soon as it detects a pilot on the Lantern carrier at a predetermined power level. Further, when the MU is in an idle mode, a Global Service Redirection message in the Lantern carrier""s paging channel directs a hard handoff to the traffic carrier of the underlay cell. When the MU leaves the underlay cell, the wireless service is handed back to the overlay cell or surrounding networks by using protocols such as Enhanced Hard Handoff (EHHO). In case an underlay cell is located close to multiple surrounding or overlay cells, a particular overlay or cell can be designated as a target cell for the MU.
Various advantages are achieved according to the present invention. For example, one advantage is that very little hardware components need to be added to available telecommunication network equipments. For instance, although the Lantern and traffic carrier use their own Channel Element Module (CEM), they can share a common Digital Control Group (DCG). In addition, the two carriers may share a common antenna configuration.